This invention relates to fluid couplings and, more particularly, to a hydraulic coupling having an input member, characterized as a bladed primary wheel or impeller, and an output member, characterized as a bladed secondary wheel or runner, confining an operating space.
A hydraulic coupling transfers energy from a drive, such as an engine or a motor, to a production machine or to the drive shaft of a vehicle. The start-up of the drive from a rest position is eased by the use of a hydraulic coupling. During the drive, for example, an internal combustion engine, rotates only at its idling speed, the torque transferred from the hydraulic coupling should be minimal so that the production machine or vehicle remains at rest.
The invention relates exclusively to hydraulic couplings constructed such that they can be operated by a constant quantity of fluid, i.e., with the coupling at rest, its inner chamber is filled by a definite volume of working or operating fluid. The volume remains unchanged during operation. Another type of hydraulic coupling utilizes an external circulatory flow arrangement which permits the degree of filling in the coupling to be varied during operation. In this way, during start-up of the drive, or during idling, the filling degree of the hydraulic coupling can be kept very low. The additional expense for the external flow circulation, however, is worthwhile only in the case of high power levels.
A hydraulic coupling is disclosed by Voith reference manual Cr 137 "Voith-Turbokupplungen fur Verbrennungskraftmaschinen" (Voith Turbocouplings for Internal Combustion Engines) which has a blade free ram space set radially within the blading of a primary wheel. This coupling, which has an especially short construction, is preferred in farm tractors, earth-moving machines and similar equipment. During normal operation, i.e. with the motor rotating at its nominal speed, slip is very low, on the order of two percent. If the drive rotation (the primary rotation rate) is reduced, a part of the flow is transferred to the ram space such that the slippage increases noticeably, i.e., the secondary rotation rate drops off to a greater extent than the primary rotation rate. In the motor idling mode, the torque transferred from the coupling is reduced to a residual amount, the so-called idling run or drag moment. In the applications named above, the idling run moment of the known coupling is small enough. However, it is still too high for the more easily starting production machines or vehicles. Thus, the secondary side begins to rotate in undesired fashion even though the drive is idling.
Attempts have been made to prevent the undesired movement of the secondary side. U.S. Pat. No. 2,347,121, for example, discloses the use of gate elements which, during motor idling, swing into the flow path of the operating fluid so as to disrupt the flow of the fluid and thus reduce the torque transferred between the coupling elements. The utilization of these gate elements, also characterized as mobile valves, substantially increases fabrication cost.